The recent growing concerns about environmental issues have led to studies on methods of reducing the use of materials derived from petroleum resources in various technical fields. Tires commercially available today contain, for example, about 20% by mass of synthetic rubber, about 20% by mass of carbon black, and other materials such as a softener and synthetic fibers, which means that they contain at least about 50% of petroleum-derived materials as a whole. Thus, reducing the dependence on petroleum-derived materials and improving the fuel efficiency of tires are both considered to be important environmental technologies to be developed in tires.
Meanwhile, improvements in the performance of cars are requiring tires to have high handling stability. In order to obtain tires with high handling stability, the hardness of the bead apex rubber needs to be increased. However, if a large amount of reinforcing filler (e.g. carbon black) is added to increase the hardness of the bead apex rubber, then problems of deterioration of processability and fuel efficiency appear. To solve these problems, rubber compositions for bead apexes containing additives such as phenolic resin have been developed.
Patent Literatures 1 and 2 disclose rubber compositions containing a thermosetting phenolic resin or a lignophenol derivative as a rubber reinforcing ingredient, as well as pneumatic tires formed from the rubber compositions. However, a part of the materials for the synthesis of the thermosetting phenolic resin of Patent Literature 1 and of the lignophenol derivative of Patent Literature 2 are petroleum-derived materials. Hence, these rubber compositions still have room for improvement in terms of the dependence on petroleum resources.
It is also described that the lignophenol derivative of Patent Literature 2 improves handling stability without an increase in rolling resistance by enhancing E* (complex elastic modulus) while keeping tan δ (loss tangent) at about 70° C. low. Still, the hardness and the handling stability of the resulting rubber are considerably low compared to general-purpose thermosetting phenolic resins. Thus, the lignophenol derivative is not considered to be sufficiently effective in increasing the hardness and rigidity of the rubber.
As described above, the conventional techniques still have room for improvement in terms of improving handling stability and the fuel efficiency while sufficiently reducing the dependence on petroleum resources.